Node.js is a platform built on Chrome's JavaScript runtime
for easily building fast, scalable network applications. Node.js
uses an event-driven, non-blocking I/O model that makes it
lightweight and efficient, perfect for data-intensive real-time
applications that run across distributed devices.

Node.js in the Industry

Node gives Azure users the first end-to-end JavaScript
experience for the development of a whole new class of real-time
applications.
Claudio CaldatoPrincipal Program Manager, Microsoft Open Technologies, Inc.

Node’s evented I/O model freed us from worrying about locking
and concurrency issues that are common with multithreaded async
I/O.
Subbu AllamarjuPrincipal Member, Technical Staff

On the server side, our entire mobile software stack is
completely built in Node. One reason was scale. The second is
Node showed us huge performance gains.
Kiran PrasadDirector of Engineering, Mobile

Node.js is the execution core of Manhattan. Allowing
developers to build one code base using one language – that is
the nirvana for developers.
Renaud WalduraSr. Product Manger, Cocktail

An example: Webserver

This simple web server written in Node responds with "Hello World" for every request.